Printed wiring boards have made great progress over this half century, which have led to their use in almost all electronic devices today. With increasing needs for downsizing and higher performance of electronic devices in recent years, higher-density packaging of mounted components and higher frequencies of signals have progressed, thus miniaturization (finer pitch) of conductor patterns and adaptation to high frequencies are needed in printed wiring boards. In particular, if an IC chip is mounted on a printed wiring board, a finer pitch of L (Line)/S (Space)=20 μm/20 μm or less is required.
A printed wiring board is firstly manufactured as a copper-clad laminate obtained by bonding a copper foil and an insulating substrate, which mainly include a glass epoxy substrate, a BT resin and a polyimide film. The bonding is performed by using a method of laminating an insulating substrate and a copper foil and heating and pressuring the resultant (lamination process), or a method of applying a varnish which is a precursor of an insulating substrate material to one side of a copper foil having a cover layer, and then heating and curing the resultant (casting process).
The thickness of a copper foil used for a copper-clad laminate is made thinner in association with a finer pitch, in such a way that the foil thickness has become 9 μm, further 5 μm or less. However, when the foil thickness is 9 μm or smaller, handling ability is greatly deteriorated in forming a copper-clad laminate by the afore-mentioned lamination process or casting process. To deal with the problem, carrier-attached copper foils have emerged utilizing a thick metal foil as a carrier, and having an ultra-thin copper layer formed thereon via a release layer. General methods of using the carrier-attached copper foil include bonding the surface of the ultra-thin copper layer to an insulating substrate, bonding the resultant by thermocompression, and then detaching the carrier via the release layer, as disclosed in Patent Literature 1, etc.
In the production of a printed wiring board using the carrier-attached copper foil, a typical method of using the carrier-attached copper foil includes firstly laminating the carrier-attached copper foil to an insulating substrate, and then detaching the carrier from the ultra-thin copper layer. Next, a plating resist formed of a photo-curable resin is provided on the ultra-thin copper layer exposed as a result of detaching the carrier. Next, predetermined areas of the plating resist are cured by exposing the areas to light. Subsequently, the uncured plating resist at unexposed areas are removed, and then an electrolytic plating layer is provided at the areas in which the resist is removed. Next, the cured plating resist is removed, thereby obtaining the insulating substrate having a circuit formed thereon, and this is used to produce the printed wiring board.